1. Field of the Invention
The present invention relates to a measurement device, more particularly to a measurement device for measuring concentration of a target substance in a liquid sample.
2. Description of the Related Art
Referring to FIG. 1, a conventional measurement system includes a driving unit 1 for generating a driving voltage, a current-type sensor unit 2 responsive to the driving voltage for generating a sensor current that is related to a concentration level of a target substance in a liquid sample 3, and a processing unit 4 for determining the concentration level of the target substance according to a result of integration of the sensor current generated by the current-type sensor unit 2.
Referring to FIG. 2, according to a conventional configuration disclosed in U.S. Pat. No. 5,682,884, the driving unit 1 generates the driving voltage 2, which herein is a direct current voltage with a non-varying magnitude, as the current-type sensor unit 2 contacts the liquid sample 3. Theoretically, the sensor current has a magnitude that varies in a positive relation to the concentration level of the target substance. However, in practice, noise components in the sensor current may lead to reduced accuracy in the concentration level determined by the processing unit 4.
Take measurement of blood sugar as example, where the liquid sample 3 is a blood sample and the current-type sensor unit 2 is a glucose sensor. For a period of time after the driving unit 1 starts generating the driving voltage, the current-type sensor unit 2 reacts to both glucose and non-glucose substances in the liquid sample 3. Reaction of the current-type sensor unit 2 to the non-glucose substances may contribute to the noise components in the sensor current.
Referring to FIG. 3, in view of the above, U.S. Pat. No. USRE36268 discloses a conventional configuration in which generation of the driving voltage by the driving unit 1 is delayed relative to the configuration disclosed in U.S. Pat. No. 5,682,884, such that the adverse effect of the noise components in the sensor current is relatively reduced.
Referring to FIG. 4, according to a conventional configuration disclosed in U.S. Pat. No. 5,620,579, generation of the driving voltage takes place as the current-type sensor unit 2 contacts the liquid sample 3, and is subsequently paused for a predetermined time before being resumed. In this configuration, the processing unit 4 determines the concentration level of the target substance after generation of the driving voltage is resumed, thereby reducing the adverse effect of the noise components in the sensor current.
In each of the abovementioned conventional configurations, since the driving voltage is a direct current voltage, and the concentration level is determined based on a result of integration of the sensor current, accuracy of the concentration level determined by the processing unit 4 depends critically on stability of the current-type sensor unit 2 when applied with a direct current voltage. That is, if the current-type sensor unit 2 has poor stability when applied with a direct current voltage, accuracy of the concentration level thus determined will be poor.